In a wireless communication system using multi-hop communication such as an ad-hoc network, there is a problem in that when only one channel is available and one wireless relay station apparatus is shared by a plurality of communication flows, since the wireless relay station apparatus alternately relays packets of the communication flows, the wireless relay station apparatus may become a bottleneck of system throughput.
As a means for solving this problem, there has been proposed technology in which the wireless relay station apparatus demodulates the packets of the respective communication flows into bit signals, performs network coding (NC) on sequences of the demodulated bit signals, and collectively transmits the sequences of the network coded bit signals to a wireless communication apparatus of a destination station (Non-patent Documents 1 and 2). The network coding indicates coding for superposing the packets of the respective communication flows through linear coding based on a predetermined generating formula.
The wireless communication apparatus of the destination station performs linear decoding based on the generating formula on a packet of a relayed signal received from the wireless relay station apparatus, thereby acquiring a desired packet. In this way, the wireless relay station apparatus superposes the packets of the plurality of communication flows and performs a relay function by collectively transmitting the superposed packets, so that it is possible to reduce the number of transmissions of the wireless relay station apparatus and to shorten the time until the communication of the wireless communication system is completed. As a result, it is possible to improve the throughput of the wireless communication system.
Hereinafter, performing the network coding on a packet will be referred to as NC encoding and decoding an NC encoded packet will be referred to as NC decoding.
A description will be given of the case in which NC technology is applied to Alice-and-Bob topology, which is the topology of the simplest multi-hop communication, and bitwise exclusive OR (XOR) is used as an example of linear coding. Furthermore, a network coded packet is defined as an NC packet, and a packet before network coding and a non-network-coded packet are defined as a native packet.
Hereinafter, with reference to FIG. 9A and FIG. 9B, packet communication performed by a wireless communication system 900 with the Alice-and-Bob topology will be described. FIG. 9A is a diagram illustrating the configuration of the wireless communication system 900. FIG. 9B is a flowchart illustrating a process of a wireless relay station apparatus 93 provided in the wireless communication system 900.
It is to be noted that in FIG. 9A, “A” denotes a signal which is transmitted from a wireless terminal station apparatus 91 (Node 1) at a time T1, “B” denotes a signal which is transmitted from a wireless terminal station apparatus 92 (Node 3) at a time T2, and “A+B” denotes a signal which is transmitted from the wireless relay station apparatus 93 (Node 2) at a time T3.
More specifically, FIG. 9A is a diagram illustrating the outline of communication in the wireless communication system 900 with the Alice-and-Bob topology. In this topology, the wireless terminal station apparatuses 91 and 92 (Nodes 1 and 3) arranged at both ends perform bi-directional communication therebetween through the wireless relay station apparatus 93 (Node 2). The following description will be given on the assumption that transmission of each node is performed at a predetermined time slot, and signals transmitted from the respective nodes do not interfere with one another. FIG. 9B is a flowchart illustrating a relay process of the wireless relay station apparatus 93. The size of a packet A transmitted from the wireless terminal station apparatus 91 is 1000 bytes and the size of a packet B transmitted from the wireless terminal station apparatus 92 is 700 bytes. Furthermore, a description will be given of the case in which a coding rate RA of the packet A and a coding rate RB of the packet B that are required for satisfying predetermined communication quality are 2/3 and 1/2, respectively.
It is to be noted that a packet corresponds to information to be transmitted, a modulation symbol corresponds to a signal obtained by modulating (mapping) a bit signal included in a packet or the like in a baseband, and an RF (Radio Frequency) signal corresponds to a signal obtained by up-converting the modulation symbol into an RF band.
First, at the time T1, the wireless terminal station apparatus 91 transmits an RF signal A of the packet A to the wireless relay station apparatus 93 and stores the packet A. The wireless relay station apparatus 93 receives the RF signal A transmitted by the wireless terminal station apparatus 91, and stores a packet A obtained by decoding the received RF signal A.
Next, at the time T2, the wireless terminal station apparatus 92 transmits an RF signal B of the packet B to the wireless relay station apparatus 93 and stores the packet B. The wireless relay station apparatus 93 receives the RF signal B transmitted by the wireless terminal station apparatus 92, and stores a packet B obtained by decoding the received RF signal B (step S901).
Then, at the time T3, the wireless relay station apparatus 93 compares a packet length LA (1000 bytes) with a packet length LB (700 bytes), appends zero padding with a data length of |LA-LB| (300 bytes) to an end of an information bit sequence of the packet B with a shorter packet length, and allows the packet lengths of the packet A and the packet B to coincide with each other (step S902). Next, the wireless relay station apparatus 93 performs NC encoding on the packet A and the packet B through XOR to generate one NC packet C (step S903), and performs error correction encoding with a coding rate of 1/2 on the generated NC packet C (step S904). In addition, the wireless relay station apparatus 93 performs multicast transmission of an RF signal C to the wireless terminal station apparatuses 91 and 92 (steps S905 and S906), wherein the RF signal C is obtained by modulating and up-converting the error correction encoded NC packet C.
Through the multicast transmission, a plurality of wireless terminal station apparatuses are designated as destination stations in header information, so that the same information is simultaneously transmitted to the plurality of destination stations using the multicast property of wireless communication. Furthermore, the packet lengths of the packet A and the packet B are included in the header information together with information indicating the destination stations. The wireless terminal station apparatuses 91 and 92 acquire the packet lengths from the header information of the received RF signal C.
Upon receipt of the RF signal C transmitted from the wireless relay station apparatus 93, the wireless terminal station apparatus 91 adjusts an output level through AGC (Auto Gain Control) using the received RF signal C, and down-converts and demodulates the received RF signal C to acquire an NC packet C. When the packet length of the NC packet C is longer than the packet length of the stored packet A, the wireless terminal station apparatus 91 performs zero padding corresponding to the difference between two packet lengths on the packet A to allow the packet length of the zero-padded packet A to coincide with the packet length of the NC packet C, and performs NC decoding through an XOR operation on the zero-padded packet A and the NC packet C to acquire a packet B.
In contrast, when the packet length of the NC packet C coincides with the packet length of the stored packet A, the wireless terminal station apparatus 91 performs NC decoding through an XOR operation on the NC packet C and the packet A. Then, the wireless terminal station apparatus 91 acquires the packet B from a packet obtained by the NC decoding based on the packet length of the packet B included in the header information.
Similarly to the wireless terminal station apparatus 91, the wireless terminal station apparatus 92 also demodulates the NC packet C from the RF signal C. When the packet length of the NC packet C is longer than the packet length of the stored packet B, the wireless terminal station apparatus 92 performs zero padding corresponding to the difference between two packet lengths on the packet B so that the packet length of the zero-padded packet B coincides with the packet length of the NC packet C, and performs NC decoding through an XOR operation to acquire a packet A. In contrast, when the packet length of the NC packet C coincides with the packet length of the stored packet B, the wireless terminal station apparatus 92 performs NC decoding through an XOR operation on the NC packet C and the packet B. Then, the wireless terminal station apparatus 92 acquires the packet A by deleting a portion appended through the zero padding from a packet obtained by the NC decoding based on the packet length of the packet A included in the header information.
As described above, the wireless relay station apparatus 93 collectively performs multicast transmission of the packets received from the wireless terminal station apparatuses 91 and 92, thereby transmitting the packet B to the wireless terminal station apparatus 91 and transmitting the packet A to the wireless terminal station apparatus 92 using three time slots reduced by one time slot, as compared with the case in which packets are individually relayed.